Extended surface area substrate particles, such as activated carbon, alumina, zeolites, and the like, are widely used in air filtration because of their ability to remove a wide range of contaminants from the air. The highly porous structure of these materials provides a high surface area that is very suitable for filtration purposes. In the case of activated carbon, the porosity results from controlled oxidation during the “activation” stage of manufacture.
The ability of such carbon to remove contaminants from air generally involves direct absorption and depends on molecular-scale interactions between a gaseous molecule and the carbon surface. The extent of this interaction may depend upon factors including the physical and chemical surface characteristics of the carbon, the molecular shape and size of the gaseous compound, the concentration of the gaseous compound in the gas stream to be filtered, residence time in the carbon bed, temperature, pressure, and the presence of other chemicals. As a rule of thumb, for a single contaminant, the extent of adsorption is primarily dependent on boiling point. In general, the higher the boiling point, the greater the capacity of carbon to remove the chemical.
Accordingly, unimpregnated carbon does not have a great capacity by itself for removal of lower boiling point gases (including reactive gases) or vapors from air. Treatments have been devised in which chemicals are incorporated into the carbon to provide improved contaminant removal capabilities. These treatments are generally known as “impregnation” methods, and the result of such treatment is an “impregnated” carbon.
The National Institute for Occupational Safety and Health (NIOSH) sets standards for respirators and respiratory protection from Chemical, Biological, Radiological, and Nuclear (CBRN) Agents. NIOSH CBRN standards for operational and escape type filters typically require that an approved device remove biological and other particulates, as well as a list of 10 gases selected to represent families of toxic compounds. The 10 NIOSH gases are sulfur dioxide (SO2), hydrogen sulfide (H2S), formaldehyde (H2CO), ammonia (NH3), hydrogen cyanide (HCN), cyanogen chloride (ClCN, or CK), phosgene (COCl2), cyclohexane (C6H12), nitrogen dioxide (NO2) and phosphine (PH3). Typically, filters meeting NIOSH standards have been constructed using a carbon capable of removing all such gases, or by using layers of carbons that collectively remove all of the listed classes of compounds. Usually one of the listed gases drives the need for increased amounts of granular sorbent material. In the case of current carbon technologies, this gas is often ammonia or sulfur dioxide.